Implement avalanche as an option/modifier/property of the hashing
functions.

Summary:
Avalanche is supported not as a native hashing choice, but a modifier
on the hashing function.

The default values were selected for backward compatibility with previous releases.

Examples
  (default) hash-type map-based
	Map based hashing using sdbm without avalanche

  (default) hash-type consistent
	Consistent hashing using sdbm with avalanche

Additional Examples:

  (a) hash-type map-based sdbm
	Same as default for map-based above
  (b) hash-type map-based sdbm avalanche
	Map based hashing using sdbm with avalanche
  (c) hash-type map-based djb2
	Map based hashing using djb2 without avalanche
  (d) hash-type map-based djb2 avalanche
	Map based hashing using djb2 with avalanche
  (e) hash-type consistent sdbm avalanche
	Same as default for consistent above
  (f) hash-type consistent sdbm
	Consistent hashing using sdbm without avalanche
  (g) hash-type consistent djb2
	Consistent hashing using djb2 without avalanche
  (h) hash-type consistent djb2 avalanche
	Consistent hashing using djb2 with avalanche
Do not clear the hash function modifier bit
Minor documentation changes in configuration.txt

Summary:
Documentation for hash-type in configuration.txt Largely done, but
needs to be proof read and the section of WT6 completed.
Merge pull request #3 from maddalab/apply_avalanche_only_when_requested

Apply avalanche only when requested